牙釉质细胞外基质蛋白主要有以下几大类（）
A.釉原蛋白
B.釉蛋白
C.非釉原蛋白
D.蛋白酶
E.非金属蛋白酶
参考答案：A, C, D
Mr White ______ a lot of money buying a new car.[ ]
A．cost
B．took
C．paid
D．spent
______ Lin Feng has to work late, she always wears a smile on her face.
A．Because
B．If
C．Until
D．Though
